U.S. Pat. No. 4,286,237 illustrates a device using a voltage controlled oscillator (VCO) and a phase lock loop (PLL) in a frequency modulated (FM) system. The reference device differs from the present invention in that: (1) there is no disclosure of the FM being generated by digital means; (2) the PLL is operable only when the modulation is zero, whereas the PLL in the preesnt invention operates continuously; and (3) the device does not use the PLL to place the signal within a higher frequency range.
The device illustrated in U.S. Pat. No. 4,387,351 uses a VCO and an automatic frequency control (AFC) loop in an FM system. The reference device differs from the present invention in that: (1) there is no disclosure of the FM being generated by digital means; and (2) the PLL is not used to place the signal at a higher frequency range.
Other references are U.S. Pat. Nos. 3,622,913 and 4,074,209.